A sealing material protects semiconductor ICs from moisture, dust and the like contained in the air and facilitates handling of semiconductor ICs. The epoxy resin sealing material is mostly used now. Conventionally, as the flame retardant for an epoxy resin sealing material, a halogenated epoxy resin or a combination of a halogenated epoxy resin and an antimony trioxide has been employed. In recent years, however, global environmental pollution problems and health hazards have emerged, and the demands for non-halogen base flame retardants have increased. And, halogen compounds of chlorine, bromine, etc. and antimony trioxide tend not to be used.
As a non-halogen base flame retardant, red phosphorus is a promising flame retardant, but it has disadvantages that a small amount of phosphine gas is produced by a reaction of red phosphorus and moisture in the air and oxo acid of phosphorus elutes from the red phosphorus surface.
When oxo acid of phosphorus eluted from the red phosphorus surface comes into contact with a semiconductor IC circuit, aluminum wiring is corroded, becoming a cause of lowering the reliability. Reduction of the eluted ions was a serious issue when the red phosphorus base flame retardant is used for the epoxy resin sealing material.
A lot of studies have been made on the problem of generation of phosphine from red phosphorus, and there have been proposed various methods of lowering the volume of phosphine generated by coating the surface of red phosphorus with hydroxide of inorganic metal such as aluminum or titanium, or coating with an organic compound such as a phenol resin or a melamine resin, or dual-coating with an inorganic compound and an organic compound.
But, oxo acid of phosphorus elutes from the surface of red phosphorus although it is coated as described above, and it was difficult to use the surface-coated red phosphorus in fields where laminated sheets, semiconductor sealing materials, etc. are required to have electric reliability. For example, when a red phosphorus base flame retardant is used for an epoxy resin for semiconductor sealing, there is a problem that a sealing material reliability test reveals that IC circuits are corroded with oxo acid of phosphorus eluted from red phosphorus resulting in lowering the reliability.
One of causes of the elution of oxo acid of phosphorus from red phosphorus is assumed that even if a method of coating the surface of red phosphorus particles is appropriate, when an epoxy resin sealing material is produced by mixing and blending red phosphorus with other materials, e.g., an epoxy resin, a phenol resin base curing agent, a silica filler, etc., the coating layer on red phosphorus is peeled because red phosphorus particles are exposed to a mechanical frictional force and a shearing force, and red phosphorus comes into direct contact with water at peeled portions to induce the elution of oxo acid of phosphorus. In such a case, the sealing material often fails the reliability test.
To provide a red phosphorus base flame retardant for a semiconductor sealing epoxy resin, there have been proposed for example a method of using red phosphorus which is coated its surface with a phenol resin and further coated with an epoxy-silane-coupling agent and an amino-silane-coupling agent and also melamine polyphosphate (Japanese Patent Application Laid-open Print No. 10-182940), a method of using a red phosphorus base flame retardant which has the surface layer of red phosphorus coated with TixOy (x, y are a positive number, x:y=1:2 to 1:4) (Japanese Patent Application Laid-open Print No. 7-173372), a method of using a red phosphorus base flame retardant having an average particle size of 2 to 8 μm and a maximum particle size of 20 μm or below which has the surface layer of red phosphorus coated with aluminum hydroxide and further coated with a phenol resin (Japanese Patent Application Laid-open Print No. 10-152599), and a method of using a red phosphorus base flame retardant which has the surface layer of red phosphorus coated with SiXOY (X, Y are a positive number, X:Y=1:2 to 1:4) (Japanese Patent Application Laid-open Print No. 7-157542).
There is also proposed a method of using a coated red phosphorus base flame retardant and an ion-trapping agent of BiOX(OH)Y(NO3)Z(X=0.9 to 1.1, Y=0.6 to 0.8, Z=0.2 to 0.4) and Mg4.3Al2(OH)12.6CO3.3.5H2O (Japanese Patent Application Laid-open Print No. 8-151427, and Japanese Patent Application Laid-open Print No. 9-227765).
But, when the coated red phosphorus base flame retardant is used to produce an epoxy resin sealing material, the coating layer on red phosphorus is peeled because the red phosphorus particles are exposed to a mechanical frictional force and a shearing force, and red phosphorus comes into direct contact with water at peeled portions to induce the elution of oxo acid of phosphorus. Therefore, there is an issue of trapping the eluted oxo acid of phosphorus to lower its action. But, the issue has not been practically solved yet because the aforesaid method of jointly using the ion-trapping agent is poor in trapping oxo acid of phosphorus.
Japanese Patent Application Laid-open Print. No. 62-21704 proposes coated red phosphorus for a flame retardant which is produced by coating the surface of red phosphorus particles with Al and/or Zn hydroxide and further coating with a thermosetting resin containing zinc hydroxide and has improved humidity resistance and corrosion resistance.
Because zinc hydroxide of Japanese Patent Application Laid-open Print No. 62-21704 is a hydrate, when stabilized red phosphorus using zinc hydroxide is used as a flame retardant for a semiconductor sealing epoxy resin, water is produced as zinc hydroxide is decomposed by a reliability test of the semiconductor sealing material at a high temperature, and red phosphorus comes into direct contact with water to elute oxo acid of phosphorus. Zinc hydroxide itself is also soluble in water at about 0.52 mg/100 ml at 18° C. It is hard to industrially obtain high-purity zinc hydroxide not containing an ionic impurity. The stabilized red phosphorus obtained by the production method of Japanese Patent Application Laid-open Print No. 62-21704 is produced by adding water-soluble metallic salt to a slurry which has red phosphorus particles dispersed into water, further adding an alkaline agent to deposit a metallic oxide or hydroxide on the surface of red phosphorus particles to obtain red phosphorus coated with an inorganic substance, after completing the reaction without washing the inorganic substance-coated red phosphorus, directly adding zinc hydroxide and a synthetic material of a thermosetting resin or its prepolymer to the slurry containing an ionic impurity having an electric conductivity of 10000 μs/cm or more, and making a polymerization reaction under the polymerization condition of the thermosetting resin monomer. The stabilized red phosphorus obtained as described above has a large amount of ionic impurities taken into coated resin at the time of the polymerization reaction, and it is hard to remove the ionic impurities from the resin by washing afterward. If washing is repeated to forcedly lower the electric conductivity, the surface coating of the red phosphorus particles is broken, increasing an eluting amount of oxo acid of phosphorus, and especially, when the stabilized red phosphorus is used as a flame retardant for the epoxy resin composition for the sealing material, electric reliability is lowered due to the elution of the ionic impurities from the resin. Thus, it had a disadvantage to be used as a flame retardant for the semiconductor sealing epoxy resin.
Japanese Patent Application Laid-open Print No. 4-130007 proposes stabilized red phosphorus which has the surface of red phosphorus particles coated with a composite film of a thermosetting resin and at least one substantially anhydrous metal oxide selected from zinc oxide, aluminum oxide and titanium oxide.
But, the stabilized red phosphorus of Japanese Patent Application Laid-open Print No. 4-130007 is used as a flame retardant for the polyolefin resin coating material for communications cables and electric cables, and when the stabilized red phosphorus is used as a flame retardant for a semiconductor sealing epoxy resin, it has a drawback that stable electric reliability cannot be proved by a reliability test of the semiconductor sealing material because an electric conductivity and the like of the stabilized red phosphorus are not specified.
In view of the aforesaid circumstances, the inventors have made a devoted study on a red phosphorus base flame retardant which is applicable to a semiconductor sealing material, and found that in a red phosphorus base flame retardant which has an anhydrous zinc compound contained in a thermosetting resin for coating red phosphorus particles or a red phosphorus base flame-retardant Composition which is formed of a mixture containing flame-retardant stabilized red phosphorus and an anhydrous zinc compound, the anhydrous zinc compound effectively reacts with oxo acid of phosphorus to form an insoluble phosphate compound to fix on its surface, the red phosphorus base flame retardant or the red phosphorus base flame-retardant composition is particularly treated by washing or the like to remove ionic impurities so to have a particular low electric conductivity, so that the red phosphorus base flame retardant or the red phosphorus base flame-retardant composition can be used suitably as a flame retardant for a semiconductor sealing resin, and completed the present invention.
Thus, it is an object of the present invention to provide a red phosphorus base flame retardant for an epoxy resin, an epoxy resin composition for a semiconductor sealing material and their production methods, which can suppress oxo acid of phosphorus from eluting from red phosphorus of the red phosphorus base flame retardant or the flame-retardant stabilized red phosphorus and can impart excellent flame retardancy to the epoxy resin for the semiconductor sealing material.
It is also an object of the present invention to provide an epoxy resin composition for a semiconductor sealing material which uses the red phosphorus base flame retardant, for an epoxy resin and an epoxy resin composition for a semiconductor sealing material and has excellent flame retardancy, humidity resistance and electric reliability, and a sealing material and a semiconductor device using the above epoxy resin composition.